The direction of aiming of the cannon with respect to the direction of the target takes into account a certain number of parameters. The distance between the cannon and the target requires an angle called the aim angle between the direction of the cannon and the direction of the traget. The relation between the aim angle and the distance is not linear. The platform supporting the turret of the cannon can take an undetermined position and it is necessary to take into account the inclination of the turning axis of the cannon. Different ammunition has a very variable behavior from the point of view of initial speed and ballistics and the characteristics of the cannon change in time. The variation of initial speed and the ballistic coefficient necessitate a correction in elevation. These parameters act angularly in a manner substantially proportional to the aim angle. It is also necessary to take into account lateral wind forces and the lateral shifting of the ammunition. These parameters necessitate corrections in bearing and the corrections are approximately proportional to the aim angle.
Different systems for firing are known. One system of firing comprises an observation telescope which can be a shielded telescope or a periscope. The generally accepted principle resides in the calculation of an aiming error between the cannon and the direct sight as a function of all the parameters of firing, after which a shift is made between the line of sight and the axis of the cannon so that the projectile strikes the target. The determination of the aiming error is effected from information from a rangefinder or determining the distance of the target. In U.S. Pat. No. 2,887,774, the deviation of the line of sight is effected by displacement of the cross-hairs of the sight. The cross-hairs are displaced by a cam.
The profile of the cam is determined by the successive values of the aim angle. The position of the cam around its axis is determined by the distance of the target given by a rangefinder. The displacement of the cross-hairs corresponds from this fact to the necessary angle of aim. The cam acts on the cross-hairs through the intermediary of a lever whose displacement of the articulation point by a mechanical system permits the introduction of secondary corrections such as the variation of initial speed and the ballistic coefficient. However, this system of firing does not permit taking into account the inclination and the secondary corrections cannot easily be taken into account.
In modern systems of conducting firing such as those described in French Pat. No. 2,016,096, the determination of the aiming error is effected with the aid of an electronic computer receiving information from a rangefinder of laser type. Generally, the firing information determined by the computer acts on an optically controlled deviator, situated in front of the telescope, which shifts both the observation and the transmission and reception beams of the laser rangefinder with respect to the cannon. The optical deviation is effected either by diasporameters or by a movable mirror. The computer apparatus not only determines the errors but also calculates their transformations into control parameters for the deviator systems. The known systems are, therefore, very complex.